1. Field of the Invention
The present invention relates in general to an apparatus for controlling an automatic transmission of a motor vehicle, and more particularly to techniques for controlling a hydraulically operated frictional coupling device which is released to achieve a shift-down action of the automatic transmission.
2. Discussion of the Related Art
There is known an automatic transmission for motor vehicle, which is adapted to effect a shift-down action by releasing a certain hydraulically operated frictional coupling device while engaging a one-way clutch, or by releasing the frictional coupling device while engaging another hydraulically operated frictional coupling device. This type of automatic transmission tends to suffer from a shifting shock upon such a shift-down action during acceleration of the vehicle with an accelerator pedal being depressed. Thus, there has been a need to minimize such a shifting shock of the automatic transmission.
An example of a known control apparatus for controlling an automatic transmission of the type described above is disclosed in JP-A-5-157167. However, this control apparatus is not satisfactory in the control of the frictional coupling device which is released to achieve a shift-down action of the automatic transmission. Described more specifically, the control apparatus is not effective enough to prevent a shifting shock of the transmission due to a torque variation during the shift-down action, or tends to cause the shift-down action to require an undesirably long time if the torque to be transmitted through the automatic transmission during the shift-down action is increased in an attempt to reduce the shifting shock. Thus, the known control apparatus does not permit the shift-down action to be achieved in a satisfactory manner.